This relates generally to electronic devices, and more particularly, to electronic devices with displays.
Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user.
Liquid crystal displays contain a layer of liquid crystal material. Display pixels in a liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the display pixel.
Substrate layers such as color filter layers and thin-film transistor layers are used in liquid crystal displays. In an assembled display, the layer of liquid crystal material is sandwiched between the thin-film transistor layer and the color filter layer.
The color filter layer contains an array of color filter elements such as red, blue, and green elements. The color filter layer provides the display with the ability to display color images.
The thin-film transistor layer contains an array of thin-film transistors that are used in controlling electric fields in the liquid crystal layer. An array of pixels is used to display images on the display. Each pixel contains a display pixel electrode and thin-film transistor circuitry for controlling the electric field that is produced in the liquid crystal layer by the electrode. The circuitry of each pixel contains a capacitor that is used to store data between successive image frames.
The array of pixels is loaded with data using vertical data lines. Horizontal control lines called gate lines are used in controlling the circuitry of the pixels in the array, so that pixels display the data provided on the data lines. With a typical arrangement, each gate line is associated with a respective row of pixels. A frame of image data may be displayed by asserting each of the gate lines in the display in sequence, so that rows of data can be loaded into the display pixels from the data lines.
Displays may be operated with a fixed refresh rate or a variable refresh rate. In a fixed refresh rate scheme, image frames are displayed at a fixed rate. The capacitors in the display pixels are used to store data on the pixels between frames. Leakage currents in the pixel circuits such as transistor leakage currents cause the data voltages on the pixel electrodes to decay. By sizing the capacitors in the display pixels appropriately for the known fixed refresh rate of the display, data voltage decay can be limited to a suitably small amount.
In variable refresh rate displays, the rate at which frames of data may be displayed on the display can be reduced when a rapid refresh rate is temporarily not needed. For example, when the only content that is being displayed on the display is static content, the refresh rate of the display can be reduced without changing the visual appearance of the display. Less power is consumed by a display when its refresh rate is lowered, so the use of variable refresh rate schemes allows an electronic device to reduce the display refresh rate whenever possible to conserve power.
Care must be taken, however, when sizing the capacitors in the display pixels of a variable refresh rate display. The slow refresh times that are required to support operation of a variable refresh rate display at low refresh rates may require the use of relatively large storage capacitances in the display pixels. If storage capacitances are too large, however, it may be difficult to load data into the display pixels effectively when the display is operated at a high refresh rate.
It would therefore be desirable to be able to provide improved pixel circuits for variable refresh rate displays.